Generally, a refrigerant compressor for an automotive air conditioning system is driven by an engine through a magnetic clutch. If the operating or driving portions of the compressor lock, the rotation load of the compressor increases excessively. This increased rotation load of the compressor can harm the engine as well as equipment disposed around and coupled to the engine.
The risk of harm to peripheral equipment connected to the engine is even higher in current automobiles because almost all equipment, such as the generator, power steering and water pumps, are driven by a single belt for space considerations. When a compressor continuously driven by an engine through an electromagnetic clutch locks, the belt can break due to the heat produced by the frictional contact between the belt and the electromagnetic clutch. Therefore, in an engine where all of the peripheral equipment are driven with the compressor by a single belt, belt breakage results in the following problems: the car is difficult to steer because the pump for the power steering stops; if the vacuum pump is driven on the same axes as the generator, the damping force of a brake with a vacuum actuator is decreased; the battery dies because the generator stops; and the engine overheats because the water pump stops. These problems make it very difficult to drive and may cause serious accidents.
To solve the above mentioned problems, the compressor is provided with a detecting device which detects rotation or motion of operating parts such as a piston, a driving axis or an inclined plate. When the compressor locks, the detecting device senses the lack of rotation or motion and causes the electromagnetic clutch to cease transmitting driving force from the engine to the compressor, thereby preventing the above mentioned problems. However, the detecting device is normally located within the compressor, creating the sealing and temperature problems around the detecting device. Also because the detecting device must be adapted to the detected portion, a proper detecting device must be designed for each type of compressor. Accordingly, it is difficult to use the same parts in different types of compressors.
One solution to the above mentioned problems is shown in the laid opened publication of Japanese Patent Application No. 58-180842. The hub of a prior art electromagnetic clutch is provided with the projection extending axially outward and a rotation detecting device disposed adjacent to the projection. In the above construction, the rotation detecting device increases compression unbalance. This decreases depressor performance and requires extra parts and space to accommodate the extra parts.